Tennis star Venus Williams withdrew from the U.S. Open on Wednesday, citing a diagnosis of Sjögren’s syndrome, a little-known autoimmune disorder that causes fatigue and joint pain. TIME Healthland asked Dr. Victoria Shanmugam, an assistant professor of rheumatology at Georgetown University, to explain the condition.

Shanmugam is not involved in Williams’ treatment but currently sees other patients with Sjögren’s (pronounced show-grens).

What is Sjögren’s syndrome?
It’s an autoimmune disease that is typically characterized by inflammation in the tear ducts and salivary glands. We’re not sure why it focuses on these tissues. Many people will have a very mild form and experience dryness of the eyes and mouth, fatigue, joint pain and myalgias [or muscle pain]. Many people will have that and nothing else. But a small proportion of people will go on to have multi–organ system disease that behaves a little more like lupus [another autoimmune disease in which the body’s own immune cells start to attack healthy tissues, particularly in the joints].

How common is Sjögren’s?
It’s not that uncommon. Estimates vary, and there may be a fair number of people who have Sjögren’s and don’t know it. So the estimates vary widely. A European study found that about 4% of the population may be affected, while a study by the Mayo Clinic found a much lower prevalence of around 1%.

It’s more common in women than in men, and often people don’t get diagnosed until middle age.

Is there a connection between Sjögren’s and lupus?
There is a degree of overlap between the two diseases. We look at the inflammatory pattern in each organ to help distinguish between Sjögren’s and lupus. For example, [in Sjögren’s] there is a particular type of inflammation in the kidney that affects how the kidneys deal with salt and acid balance, which produces a very different picture than the inflammation of the kidney’s glomerulus, which is involved in filtering waste from the blood, that we see in lupus.

Some people will have particular types of inflammation in the lung and liver that is only seen in Sjögren’s, which can sometimes go on to become lupus. But the fact that [Williams] was diagnosed with Sjögren’s doesn’t necessarily mean she will go on to develop lupus.

What causes Sjögren’s?
That’s the million-dollar question in rheumatology. We think people probably have some kind of genetic predisposition — but we don’t know for sure — and some environmental factor sets off the immune system to trigger the disease. It’s not well understood why this particular person [will get diagnosed] and why now at this point in their life. But we do know that stress makes immune diseases worse.

Can Sjögren’s be treated?
Yes. We’ll treat people dependent on where they are on the disease spectrum. For the milder cases, we have topical [options] to treat dryness of the eyes and mouth. These can suppress the immune system locally in the eye to reduce inflammation. We also warn patients to get regular dental checkups.

For slightly more severe disease, we use disease-modifying drugs that are similar to treatments for lupus. Hydroxychloroquine, an anti-malaria [drug] can be helpful for joint pain, muscle aches and fatigue. If there is more severe inflammation involving the lung, kidney or liver, we can pick more potent immune-suppressing medications such as steroids or methotrexate to really suppress the immune system.

Do you think Williams will be able to return to playing professional tennis?
Her decision to take a little time off and recover and get her health under control is a reasonable decision. There is no reason to think that having this diagnosis shouldn’t allow her to continue to be active in the future. Many many people with Sjögren’s lead very full and active lives. And in most cases, it doesn’t affect their ability to hold down a full-time job and maintain all their usual activities.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.